Method and apparatus for continuous casting of ingots



May 7, 1968 5. R. GARDNER METHOD AND APPARATUS FOR CONTINUOUS CASTING OF INGOTS 2 Sheets-Shes:

Original Filed June '7, 1963 INVENTOR. GEORGE R. GARDNER Attorney May 7, 1968 G. R. GARDNER 3,331,741

METHOD AND APPARATUS FOR CONTINUOUS CASTING OF INGOTS Original Filed June 7, 1963 2 Sheets-Sheet 2 Flt-L 4..

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GEORGE E. GARDNER A! farney United States Patent 3,381,741 METHOD AND APPARATUS FOR CONTINUOUS CASTING OF INGOTS George R. Gardner, Berea, Ohio, assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania Continuation of application Ser. No. 286,349, June 7, 1963. This application Apr. 13, 1966, Ser. No. 542,424 14 Claims. (Cl. 164-73) ABSTRACT OF THE. DISCLOSURE In the continuous casting of aluminum or aluminum alloy ingots wherein a body of molten metal is maintained adjacent to a chilled continuous casting mold and continuously fed thereinto, the surface finish of the ingot produced can be markedly improved by extracting heat in two zones. The first zone is established by a surface disposed laterally inward of the surface describing the second zone. The length of the first zone is considerably shorter than that of the second zone and the inward projection of the first zone is generally less than 0.15 inch. The length of the first zone generally varies between about and inch whereas the length of the second zone may be 1 to 2 inches or more.

This invention relates to the continuous casting of ingots and has particular reference to casting ingots in molds in which extraction of heat is controlled where the molten metal comes in contact with the mold wall, and this application is a continuation of Ser. No. 286,349 filed June 7, 1963, now abandoned. The invention is especially adapted to the casting of the light metals aluminum and magnesium and the alloys wherein these metals predominate.

Numerous molds and methods have been proposed for the continuous casting of ingots. One of the most common types of molds that have been used is of stationary tubular design that may or may not be attached to a furnace or other metal holding receptacle. In this type the mold is drastically chilled, usually by a liquid coolant, and generally a liquid coolant is applied to the ingot emerging fro-m the mold. In the case of casting light metals the ingot surfaces are often rough because of variations in cooling conditions, one of the common manifestations of this being the occurrence of bands or rings of exuded alloy constituents that are usually referred to as liquation. The bands or rings tend to form laps and under severe conditions even cracks extending into the ingot. In some cases it is necessary to remove the liquated zone by machining or some other suitable method in order to obtain a surface of uniform composition and free from defects. Such surface preparation is highly desirable where the ingot surface is being extended by rolling, forging and similar metal working operations.

Variation in the surface tension of the liquid metal just ahead of the line of contact with the mold also has an important effect upon the character of the frozen metal surface. When the surface tension fluctuates, the space between the metal and the mold is altered and freezing is momentarily accelerated or retraded with a resultant irregularity in the surface of the ingot such as referred to above. In addition, the variation in rate of freezing affects the degree of exudation or liquation of the alloy.

The surface quality of an ingot can be greatly improved, I have discovered, by increasing the area of contact between the mold wall and metal where freezing begins and extracting heat in a step-wise manner.

It is an object of this invention to provide a method ice of continuously casting an ingot by which the surface irregularities are reduced to a minimum. Another object is to provide a method of continuously casting ingots which effects a control over the temperature of the surface of the metal as freezing begins. Still another object is to provide a method of continuously casting aluminum base alloy ingots which exhibit substantially no liquation on their surface. In respect to apparatus it is an object of this invention to provide a mold and associated structure for the continuous casting of ingots that permits temperature control over the molten metal as it begins to change to the solid state. A further object is to provide a simple mold structure for continuous casting of ingots with a built-in means of controlling the temperature of the molten metal at the head end of the mold. Another object is to provide a mold for the continuous casting of light metal ingots which effects a drastic chill of the metal over a relatively short distance.

These and other objects and advantages will become apparent from the following description and accompanying drawings wherein FIG. 1 is a plan view of a unit for casting circular ingots in a vertically positioned mold;

FIG. 2, taken on line 22 of FIG. 1, is a sectional view showing the unit containing molten metal and the descending ingot;

FIG. 3 is an enlarged view showing details of the mold structure where it adjoins the overlying refractory member;

FIG. 4 is a sectional view of a horizontally disposed mold and associated molten metal resrevoir;

FIG. 5 is an end view taken on line 55 of FIG. 4; and

FIG. 6 is an enlarged view of the mold structure where it adjoins the refractory wall member adjacent the gate opening in the wall member.

My invention is based on the discovery that the surface quality of ingots is greatly improved by increasing the extraction of heat in the region Where freezing of the molten metal begins and this is accomplished in steps. The extraction of heat is increased by increasing the area of contact between the molten metal and the mold wall, other conditions remaining unchanged. To achieve this result the surface of the liquid metal is not substantially chilled as it approaches the chilled mold wall but heat is first intentionally extracted in an intermediate zone before the metal comes in contact with the drastically chilled mold wall. In this manner the metal remains in a plastic or semi-fluid condition and in contact with the mold wall so that transfer of heat from metal to mold is facilitated. The transition from liquidto solid states of the surface of the metal takes place within a very short distance but it is within that distance, I have found, that the pattern is established for the surface quality of the ingot. A preferred means of obtaining heat extraction in the intermediate zone is to place a relatively thin heat conductive member over the inner portion of the head face of the mold, the inner edge of said thin member coming in contact with the molten metal immediately adjacent to the inner mold wall surface at the head end thereof.

To carry out my invention it is essential that the molten metal be fed to the head end of the mold without substantial loss of heat and under a relatively small pressure such as that provided by a small hydrostatic head and thus assist in maintaining contact between the fluid metal and the mold wall where freezing starts. The metal should be in a liquid, freely running, form as it reaches the mold wall so that it will readily con-form to the wall and thereby improve contact between the metal and the mold. The use of a heat insulative refractory material next to the head of the metal mold is a convenient means of maintaining the molten metal at the desired temperature.

To further improve the contact between the molten metal and the mold wall there shouldbe at least some lateral movement of the surface of the body .of metal entering the mold. The lateral movement refers to a spreading out of the surface of the molten metal at the head ofthe mold, said movement being generally transverse to the direction of flow of the main body of metal. It is the surface flow of metal which is important in initiating freezing without irregularities in the surface of the ingot. The main body of metal can enter the central portion of the mold in the case of a vertical casting unit and upon freezing increase the thickness of the walls of the forming ingot as well as filling the central part of the ingot.

I have found that greatly improved surface quality of the ingots can be obtained by chilling the surface of the molten metal in step-wise fashion, at the mold wall, the initial chilling being less drastic than in the following step or steps. The most drastic chill within the mold is considered to be that provided by the coolant chilled mold walls.

In my preferred practice the chilling can be accomplished in two steps, the surface of the molten metal :being partially chilled and freezing initiated as it moves from the non-chilled zone or region to the drastically chilled zone provided by the inner mold Wall surface. Partial or intermediate chilling, as that expression is used herein, refers to cooling the molten metal to a temperature where some freezing occurs but the metal still retains a high degree of fluidity or plasticity. In the drastically chilled zone there is substantially complete solidification of the metal next to the mold wall. The partial or intermediate chilling is effected over a relatively short distance as compared to the length of the drastically chilled zone and less heat is apparently extracted in the intermediate zone than in the second or drastically chilled zone. Progressive extraction of heat in this manner develops a freezing pattern for the ingot surface that remains relatively un changed as the ingot walls become thicker and the ingot eventually is completely solidified. In contrast, aluminum alloy ingots, for example, that have been cast according to conventional practice in direct chill molds, Where there is no intermediate zone cooling, develop rough, irregular surfaces. Other work has also demonstrated that an abrupt transition from the liquid to solid state introduces variations in the frozen metal surface that are objectionable.

The walls of the ingot increase in thickness as the forming ingot progresses through the mold as more heat is extracted. To effect .a further chilling of the ingot it is generally advisable to apply coolant to the surface of the ingot as it leaves the mold. In this manner a solid ingot is formed within but a short distance of the exit end of the mold. Means are usually provided for withdrawing the ingot from the mold at a substantially constant rate.

Vertical or horizontal casting units adapted to effecting a progressive or step-wise chilling of molten metal to form ingots having high surface quality include a cham ber or other means of holding molten metal at a substantially constant temperature and delivering it to a chilled mold. The molds are generally made of any suitable metal having a relatively good thermal conductivity. For casting light metals, aluminum or copper and their alloys can be used. The chilling of the mold can be accomplished by means of applying liquid coolant to the external surface or by providing cooling chambers within the mold wall. The ingot emerging from the mold is also generally chilled by the application of coolant. At the head of the moldwhere it adjoins the metal holding container, a heat insulative refractory member is provided having an opening therethrough for transfer of molten metal to the mold, the edges of the opening projecting inwardly from the inner mold wall. Intermediate said heat insulative member and the head of the mold and extending to the inner surface of the mold Wa iS a relatively thin insert, which resembles a gasket, and preferably is in a plane normal to the axis of the mold. The insert covers only a portion of the interfacial area between the head .of the mold and the heat insulative member adjacent the inner mold wall but the inner edge of the insert is exposed to the molten metal which fills the mold. Furthermore, the edge of the insert should project slightly inwardly of the inner mold wall surface. The insert only requires a compression fitting with the head of the mold and the adjacent heat insulative refractory member, such a contact being adequate for heat transfer to the chilled mold. The compression is usually great enough to prevent leakage of molten metal into the interfacial area between the.

head of the mold and the heat insulative member. To facilitate movement of the ingot over the mold surface a lubricant is advantageously supplied at the head of the mold by suitable means. Conventional means are provided for withdrawing the ingot from the mold.

Referring to the drawings, FIGS. 1, 2 and 3 illustrate apparatus adapted to the casting of round ingots in a vertical direction. It is to be understood that other shapes can also be cast and that other types of chilled molds can be employed. In FIG. 1, the top view of a casting unit is to be seen with retaining ring 2 and associated lugs 4 for holding in place the heat insulative refractory members 6. Although various heat insulative materials can be employed which are resistant to attack by the molten metal, for casting aluminum and aluminum base alloys it has been found that an asbestos-silica composition sold under the name Marinite is suitable. This is available in the form of plates or boards and to provide a metal holder of adequate depth, it may be necessary to superpose one plate upon another as seen in FIG. 2. Molten metal is fed to the metal holding chamber through a downspout 12 attached to a source of supply, such as a trough, and forms a body of molten metal 14 in the metal holding chamber and within the mold. To effect a lateral distribution of the molten metal, the downspout 12 is closed at its bottom end and the metal is discharged through lateral ports 16 against the wall of a conventional baffle 8 supported by legs 10 extending over the top of the refractory members 6. The baffle 8 not only diverts the incoming metal in a downward direction but serves to collect dross particles and other contaminants which will float on the liquid metal.

Turning to FIG. 2, the construction of the metal holding chamber, mold and related structures is seen in detail. The downspout 12, baffle 8, retaining ring 4 and the builtup wall of heat insulative members 6 are shown in section. The inverted L-shaped aluminum mold 18 has a head flange 20 to which the heat insulative members 6 are attached. The mold is chilled by water from spray pipe 30 and the ingot descending from the mold is cooled by water from spray pipe 32. In referring to an aluminum mold it is to be understood that here and elsewhere the term is used in a generic sense and includes aluminum base alloys.

The opening in the heat insulative member for movement of the molten metal into the mold conforms to the shape of the inner mold wall and, as described below, the memberprojects inwardly from the inner mold wall.

Liquid lubricant is supplied to the head end of the mold through a passageway 28 connected to a source of supply, not shown. The passageway 28 leads to an annular channel 26 and from thence the lubricant passes through small grooves 38 to the inner mold wall surface, as is more clearly seen in FIG. 3.

The insert 22, situated in recess 24, in the head face of the mold 18, is relatively thin, and preferably has fiat faces which contact the mold 18 and the heat insulating refractory member 6 and in a plane normal to the axis of the mold. The insert extends inwardly from the mold wall surface for but a short distance, the remainder of the head face of the mold being compressed against the heat insulative member 6. The insert has a relatively small width with respect to the full width of the interface between the head of the mold and the heat insulative member. A width of about an inch is satisfactory. Although the insert is relatively small with respect to the adjoining mold and heat insulative member it is nevertheless highly effective in extracting some heat from the metal coming in contact with the inner edge thereof.

The relative positions of the heat insulative member 6, the insert 22 and the inner wall surface of mold 118 are seen more clearly in FIG. 3. The heat insulative member 6 extends inwardly of the insert 22 for a short distance 34 while the insert 22 likewise projects beyond the inner mold wall surface by a short distance 36. These slight projections are essential to controlling the surface flow of metal and permitting to spread out. With this arrangement the effects of surface tension of the liquid are overcome, that is, a large convex meniscus is not formed with an insulating air space between it and the junction of the mold with the heat insulating member. The insert projects inwardly far enough to insure contact with the surface of the molten metal. While the extent of the projection can vary with the metal being cast, it has been found in the casting of aluminum and aluminum base alloys that this should be between 0.003 and 0.15 inch. Projections of this magnitude are considered to be slight. It is also important that the insert be properly positioned in order to provide a uniform projection around the entire mold.

The insert can be made of any heat conductive material which will withstand the temperatures of the molten metal. In casting aluminum and aluminum base alloys, graphite can be employed as well as other non-metallic heat conductive materials. Instead of graphite or its equivalents, a metal may be employed providing it has a sufiiciently high thermal conductivity. It has been found that aluminum and aluminum base alloys can be used for this purpose, the chilling effect of the adjoining mold being great enough to prevent any melting. Higher melting point metals can also be used if they possess the necessary thermal conductivity.

The thickness of the insert is important for the chilling effect must be confined to a relatively small zone. For example in casting aluminum and aluminum base alloys the insert may vary in thickness between about and inch. A thinner insert has no significant effect upon the chilling of the molten metal while a thicker insert appears to promote the occurrence of liquation. The choice of thickness in individual cases will be influenced by the size of the ingot being cast but in any case the insert is relatively thin. Thickness on the order of those mentioned above are considered to represent a relatively thin insert.

The solidified ingot 40, as seen in FIG. 2 has a freezing front 42, which, it will be observed, extends below the bottom edge of the mold. This condition usually prevails in continuous casting of ingots. The solidified metal, however, begins to appear at the top of the mold and opposite the mold insert described above.

Horizontal casting of ingots is illustrated in FIGS. 4, 5 and 6. The process and the essential parts of the apparatus are the subject of co-pending application Ser. No. 286,033, filed June 6, 1963, now Patent No. 3,286,- 309. Referring to FIG. 4, a chamber 44 is employed to hold molten metal supplied to the mold 50, and one wall 46 of the chamber includes a gate 48 situated close to the inner mold wall. Since the casting unit is designed to cast round ingots, the gate 48 is circular in shape and extends over only the lower half section of the mold. The outline of the head of circular mold 64 is to be seen in dotted lines in FIG. 5. Other shapes of ingots can f course be cast in a horizontal unit.

The circular ingot mold 50 which can be conveniently made of cast aluminum where aluminum ingots are being cast, includes an annular liquid coolant chamber 52 which is supplied with Coolant through pipe 54. The

coolant is discharged from the chamber upon the emerging ingot through channels 56.

In the head face 64 of the mold, a recess 66 is provided for reception of insert 58, as is more clearly seen in FIG. 6. In addition to the insert provision is made in the head of the mold for introduction of liquid lubricant to the inner mold surface, the lubricant entering through passageway 62 to annular channel 60 and from there through grooves 68 to the mold surface. In this case, as in the one shown in FIGS. 2 and 3, the insert can serve as one side of the grooves leading to the inner mold surface.

In the horizontal casting of ingots in apparatus described above, the wall 46 extends inwardly of the inner mold wall and the surface of the molten metal remains in contact with it before reaching the insert 58. The insert also projects inwardly from the mold wall as in the case of the vertical casting unit by a margin 70 as seen in FIG. 6. The insert performs the same function as in the unit shown in FIGS. 2 and 3 and is subject to the same limitations and variations.

The ingot is withdrawn from the mold by any conventional means, not shown.

In operation the open end of the mold is filled with a starting block, molten metal introduced as the mold is being chilled. Upon filling the mold cavity, the block with frozen metal thereon is gradually withdrawn and the ingot emerges following the block. These operations are known in the art and need not be described in detail.

My invention is illustrated in the following examples:

EXAMPLE 1 An ingot was cast in a vertically disposed circular water chilled aluminum mold of the type shown above in which the mold had an internal diameter of 9 inches and a length of 1 inch. Water was sprayed on the mold and descending ingot at the rate of 26 gallons per minute while the ingot was being lowered at a rate of 4 inches per minute. The alloy employed in the test had a nominal composition of 0.7% magnesium, 0.4% silicon and balance aluminum which was melted and supplied to the metal holding chamber above the mold at 1260 to 1280 F. The heat insulative member at the top of the mold extended inwardly of the mold wall about 1.inch and the graphite insert, inch in thickness and 1 inch in width extended 0.006 to 0.009 inch inwardly of the mold wall. Under these conditions an ingot was produced which had a relatively smooth surface, showing only light ripples and having no cracks, laps or other defects which are often found on ingots cast in conventional direct chilled v vertical molds.

EXAMPLE 2 In another case another alloy was cast in a circular vertical aluminum water chilled mold which had an internal diameter of 9 inches and a length of 1% inches. The alloy had a nominal composition of 5.6% zinc, 2.5% magnesium, 1.6% copper, 0.3% chromium and balance aluminum. The heat insulative member in this instance projected inwardly of the mold wall inch While the graphite insert of the same size as that in Example 1 extended 0.006 to 0.009 inch inwardly of the mold wall. The molten metal was supplied at a temperature of 1260 to 1280 F. and the ingot was lowered at the rate of 3 /2 inches per minute. The mold and descending ingot were sprayed with water at a rate of 16 gallons per minute. The resulting ingot had the same type of surface as in Example 1, none of the common defects being present.

EXAMPLE 3 In order to determine the temperature of the graphite insert and adjacent aluminum mold a test was arranged involving use of a horizontally disposed mold of the type illustrated above. For this test the same alloy was used as employed in Example 1 and supplied to the mold at a temperature of 1255 to 1270 F. The circular cast aluminum mold had an internal diameter, of 6 inches and was 2% inches in length. The graphite insert was A1, inch in thickness and was about 1 inch in Width. Thermocouples were inserted in the graphite at the mid points of the top and bottom of the insert as Well as on the same midpoint line in the mold /8 inch away from the graphite toward the exit end of the mold. At an ingot withdrawal rate of 5 /2 inches per minute, the temperature at the top of the graphite insert was 390 F. while in the mold it ranged between 340 and 385 F. At the bottom position the temperature in the graphite was 500 F. and that in the aluminum mold was 440 F. Those temperature differences show quite clearly that the insert was not chilled as much as the mold and that it serves to extract heat in advance of the chilled mold surface. The ingot produced had a light rippled appearance Without any of the defects mentioned above.

Having thus described my invention and certain embodiments thereof, I claim:

1. Method of continuously casting an ingot of aluminum or an alloy thereof having improved surface quality comprising:

(1) maintaining a body of molten metal adjacent a chilled continuous casting mold,

(2) continuously feeding molten metal to the entrance of said chilled mold,

(3) extracting heat from, and initiating solidification of, the surface of the metal in a first heat extracting zone having a surface substantially arallel to the mold axis by continuously moving said metal surface in heat transfer relation with said first heat extracting zone surface,

(4) extracting additional heat, and further effecting solidification, in a second heat extracting zone having a surface substantially parallel the mold axis and disposed laterally outwardly of said first heat extracting zone surface, by continuously moving said surface of the metal in heat transfer relation with said second heat extracting zone surface,

(5) continuously withdrawing, at a substantially constant rate, the ingot from the mold.

2. The method according to claim 1 wherein a lubricant is continuously applied to the said moving metal surface.

3. The method according to claim 1 wherein a lubricant is continuously applied to the said moving metal surface betore it reaches the second heat extracting zone.

4. The method according to claim 1 wherein the molten metal is moved laterally outwardly and remains molten until reaching said first heat extracting zone.

5. The method according to claim 1 wherein the first heat extracting zone is short relative to the second.

6. Apparatus for the continuous casting of an aluminum or aluminum alloy ingot comprising:

(1) an open ended heat conducting mold,

(2) a chamber for holding a body of molten metal with a heat insulative refractory member adjacent the mold and having an opening therein for the passage of molten metal from said chamber into said mold,

(3) a relatively thin heat conducting portion at the mold entry having an inside surface substantially parallel to the mold axis and extending around the entire mold opening and disposed slightly laterally inwardly of, and substantially conforming to the general shape of, the remaining inside surface of said mold,

(4) means for chilling said mold.

7. Apparatus according to claim 6 including means for continuously applying lubricant to the inside surface of the mo'id.

3. Apparatus for the continuous casting of an aluminum or an aluminum alloy ingot comprising:

(1) an open ended heat conducting mold,

(2) a chamber for holding a body of molten metal with a heat insulative refractory member adjacent the mold and having an opening therein for the passage of molten metal from said chamber into said mold,

(3) a relatively thin heat conductive insert at the mold entry and in contact with the mold and the heat insulative member and having an inside surface substantially parallel to the mold axis and extending around the entire mold opening and disposed slight- 1y laterally inwardly of, and substantially conforming to the general shape of, the remaining inside surface of said mold,

(4) means for chilling said mold.

9. Apparatus according to claim 8 including means for introducing lubricantbetween said insert and said mold.

10. Apparatus according to claim 8 wherein the axis of the mold is substantially vertical and the molten metal holding chamber is situated above the entrance to said mold.

11. Apparatus according to claim 8 wherein the axis of the mold is substantially horizontal and the molten metal holding chamber is constructed to hold a molten metal level above the mold. I

12. Apparatus according to claim 8 wherein said insert is from to inch in thickness and said inside surface thereof is disposed inwardly of said remaining inside surface of said mold by from 0.003 to 0.15 inch.

13. Apparatus according to claim 8 wherein the insert is composed of graphite.

14. Apparatus according to claim 8 wherein the opening in said heat insulative refractory member extends inwardly of said inside surface of said insert.

References Cited UNITED STATES PATENTS 2,367,148 1/1945 Smart et a1. 164-82 2,983,972 5/1961 Moritz 164283 3,040,396 6/1962 Hudson 16473 3,206,808 9/1965 Robinson 164273 3,210,812 10/1965 Belwick 164-283 3,212,142 10/1965 Moritz 164282 3,286,309 11/1966 Brondyke et al. 164-4283 3,085,303 4/1963 Steigerwald 164-82 I. SPENCER OVERHOLSER, Primary Examiner.

R. D. BALDWIN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,381 ,741 May 7, 1968 George R. Gardner It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shoyp v below:

Column 2, line 31, "resrevoir" should read reservoir Coliimh8;"line' 54, "Belwick" should read Berwick Signed and sealed this 23rd day of September 1969.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward Fletcher, 11'.

Commissioner of Patents Attesting Officer 

